1. Field of the Invention
This invention pertains generally to devices for controlling speed, direction, and torque of motors, and more particularly to H-Bridge type motor controllers which are commonly used to control stepping motors.
2. Description of the Background Art
H-Bridge type motor drivers are commonly used to control the speed, direction, and torque of stepping motors. Conventional H-Bridge type motor drivers, however, exhibit a number of operational limitations such as (i) limitations on the motor driving voltage, thereby restricting very high speed operation, (ii) a narrow range of frequency over which the power driver circuit can operate, (iii) noise transferring from the power switching circuit (H-Bridge) into the motor driver logic circuitry, and (iv) susceptibility of the motor driver logic circuitry to damage in the event of failure of the power circuit.
Further, conventional H-Bridge type drivers typically utilize a power supply which powers both the H-Bridge circuity and the digital control circuitry. When the electrical power applied to the motor coil by a conventional H-Bridge controller is turned off, the magnetic field in the motor collapses and the unused energy, along with the additional energy from the motor acting as an electrical generator, flows back into the field effect transistors (FET) which are typically used to form the H-Bridge. This energy can be quite large and can damage or destroy the high side FETs. As a result, most FET manufacturers incorporate protection diodes into FETs used for driving inductive loads like motors and relays. However, while these diodes prevent failure of the FETs, they have only limited success in dissipating the excess energy. In practice, significant energy leaks over to the gate of the high side FETs and ends up interfering with the gate driving signal. As a consequence, the FETs end up being switched "on" when they should be "off", thereby causing the motor to operate less efficiently, and ultimately to prematurely stall.
Also, a common practice with conventional H-Bridge motor drivers is to use a technique called current chopping whereby the logic "on time" for each leg of the H-Bridge is limited to that necessary to raise the current level in the motor coil to a desired level. This technique involves applying a fixed voltage to the motor and then monitoring the current developed across a sensing resistor connected between the sources of the low side FETs and ground. When the current rises to the desired level, the logic circuit turns off the FET driver signals. However, while current chopping is a very useful technique, it does not alleviate two of the most serious problems encountered in switching inductive motor loads. First, it does nothing about the energy from the collapsing magnetic fields flowing back into the H-Bridge FETs. Second, it does not solve the problem of trying to match the amount of driving voltage to the motor armature characteristics at a given speed.
It is well known that, in order to make stepping motors (and other motors as well) turn at faster speeds, it is necessary to apply greater amounts of electromotive force (voltage) to overcome the increasing inductive reactance of the motor armature. Generally, the higher the voltage, the higher the possible motor speed. However, when a fixed high voltage is applied to obtain a very high speed, the motor's low speed operation is degraded by increased vibration and noise and, in the case of stepping motors, by increased likelihood of missed steps.
Additionally, the inductance of the motor and the electrical pathway between the FETs and the motor causes a voltage spike upon turn on of the circuit. This spike increases dramatically as the voltage exceeds that needed for a given speed and work load. If the spike gets too large, it can lead to failure of the FETs. In conventional controllers, this voltage spike is typically treated symptomatically; either a snubber (consisting of a low value resistor and capacitor in series) is placed across the motor coil, or a fast switching diode is placed across the FET. The snubber acts to slow the rise time of the spike, while the fast switching diodes route the spike around the FET.